The vacuum, countergravity, shell mold casting process is particularly useful in the making of thin-walled, near-net-shape castings and involves: sealing a bottom-gated mold, having a gas-permeable upper portion, to the mouth of a vacuum chamber such that the chamber confronts the upper portion; immersing the underside of the mold in an underlying melt; and evacuating the chamber to draw melt up into the mold through one or more of the gates in the underside thereof. Such a process is shown in U.S. Pat. No. 4,340,108 wherein the mold comprises a resin-bonded-sand shell having an upper cope portion and a lower drag portion sealingly mounted to the mouth of the vacuum chamber by means of spring clips. U.S. Pat. No. 4,340,108 seals the mold to the vacuum chamber atop the cope such that the parting line between the mold halves lies outside the vacuum chamber. U.S. Pat. No. 4,632,171 seals the mold to the mouth of the vacuum chamber atop the drag such that the parting line between the cope and drag falls within the vacuum chamber. U.S. Pat. No. 4,658,880 mounts the mold to the vacuum chamber by means of a plurality of reciprocable and rotatable shafts having self-tapping threads on the lower ends thereof engaging mounting sites atop the mold. Reversible motors rotate the shafts so as to screw the threads into engagement/disengagement with the mounting sites to mount/demount the mold to/from the vacuum chamber. Screwing of the threads into the mounting sites draws the mold into sealing engagement with the mouth of the vacuum chamber. Chandley, G. D. Automatic Countergravity Casting of Shell Molds, Modern Casting, October 1983, pages 29-31, mounts round molds to a round vacuum chamber wherein the inside surface of the vacuum chamber includes self-tapping threads which screw into the periphery of the round mold. U.S. patent application Ser. No. 147,863 filed Jan. 25, 1988, describes a technique for mounting the mold to the vacuum box via a plurality of T-bar keepers engaging anchoring cavities in the mold.
When the aforesaid mold-chamber arrangements are used with thin molds having more than about 400 square inches of mold confronting the vacuum chamber, there is a tendency for the molds to bow or flex into the chamber when the casting vacuum is drawn in the chamber. This flexure can destroy the mold by forming cracks therein if not total fracture (i.e., implosion) of the mold into the chamber.
The aforesaid patents/publication all disclose molds wherein the upper and lower halves are glued together. The gluing process is expensive and time consuming and elimination thereof would improve the efficiency and economics of the process. Two techniques for eliminating gluing and resisting destructive flexure of the mold are the subject of copending U.S. patent application Ser. Nos. 211,020 and 211,024 filed concurrently herewith and assigned to the assignee of this application.
It is an object of the present invention to provide improved apparatus for the vacuum, countergravity casting of shell molds including means for securing the upper mold portion into sealing engagement (sans glue) with the lower mold portion after the mold has been mounted to the mouth of the vacuum box and for resisting destructive flexure of the mold during casting. It is a further object of the present invention to provide such improved apparatus with means for automatically controlling the securing of the mold members together so as to provide such sealing and resistance without dislodging the mold from the chamber. These and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.